Fix error handling when looking for configuration files (#6676)

## What's the problem this PR addresses?

<!-- Describe the rationale of your PR. -->
<!-- Link all issues that it closes. (Closes/Resolves #xxxx.) -->

When looking for configuration files if reading a candidate throws an
exception the exception is not caught and the user gets a cryptic and
non-informative message. e.g. if you happen to have a directory named
`.yarnrc.yml` in the root of your project every yarn command will result
with this message:
```
Internal Error: EISDIR: illegal operation on a directory, read
Error: EISDIR: illegal operation on a directory, read
```
which doesn't include the file.

## How did you fix it?

<!-- A detailed description of your implementation. -->

This change moves the reading operation into the try clause and handles
read errors in the same way parsing errors are handled.

## Checklist

I did it from GH online editor so I can't run stuff. Sorry.
Hopefully someone can pick it up it would save the next poor soul a few
hours.

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---------

Co-authored-by: Maël Nison <[email protected]>

Author: yannayl

.yarn/versions/a0ea31c2.yml

@@ -0,0 +1,34 @@
+releases:
+  "@yarnpkg/cli": patch
+  "@yarnpkg/core": patch
+
+declined:
+  - "@yarnpkg/plugin-compat"
+  - "@yarnpkg/plugin-constraints"
+  - "@yarnpkg/plugin-dlx"
+  - "@yarnpkg/plugin-essentials"
+  - "@yarnpkg/plugin-exec"
+  - "@yarnpkg/plugin-file"
+  - "@yarnpkg/plugin-git"
+  - "@yarnpkg/plugin-github"
+  - "@yarnpkg/plugin-http"
+  - "@yarnpkg/plugin-init"
+  - "@yarnpkg/plugin-interactive-tools"
+  - "@yarnpkg/plugin-link"
+  - "@yarnpkg/plugin-nm"
+  - "@yarnpkg/plugin-npm"
+  - "@yarnpkg/plugin-npm-cli"
+  - "@yarnpkg/plugin-pack"
+  - "@yarnpkg/plugin-patch"
+  - "@yarnpkg/plugin-pnp"
+  - "@yarnpkg/plugin-pnpm"
+  - "@yarnpkg/plugin-stage"
+  - "@yarnpkg/plugin-typescript"
+  - "@yarnpkg/plugin-version"
+  - "@yarnpkg/plugin-workspace-tools"
+  - "@yarnpkg/builder"
+  - "@yarnpkg/doctor"
+  - "@yarnpkg/extensions"
+  - "@yarnpkg/nm"
+  - "@yarnpkg/pnpify"
+  - "@yarnpkg/sdks"

packages/yarnpkg-core/sources/Configuration.ts

@@ -1406,15 +1406,15 @@ export class Configuration {
       const rcPath = ppath.join(currentCwd, rcFilename as PortablePath);
 
       if (xfs.existsSync(rcPath)) {
-        const content = await xfs.readFilePromise(rcPath, `utf8`);
-
         let data;
+        let content;
         try {
+          content = await xfs.readFilePromise(rcPath, `utf8`);
           data = parseSyml(content) as any;
         } catch (error) {
           let tip = ``;
 
-          if (content.match(/^\s+(?!-)[^:]+\s+\S+/m))
+          if (content?.match(/^\s+(?!-)[^:]+\s+\S+/m))
             tip = ` (in particular, make sure you list the colons after each key name)`;
 
           throw new UsageError(`Parse error when loading ${rcPath}; please check it's proper Yaml${tip}`);